They hired you knowing full well that you were pregnant already so legally they can't let you go over a Dr. note saying they have to find you other duties to do other than lifting and bending.
If they don't comply then I would contact the labour board.
Thay can however ask you to take a leave of absence if they cannot provide you with duties that comply with your Doctor's orders, in this case you would still have a job to come back to once your maternity leave is over.
I just recently left my job, do to extreme stress and unfortunately do not qualifyfor EI as I've only been back to work a month after maternity leave. However a friend of mine works in the legal system and has pointed me to the Emplotment Standards Act and had my employer said or done anything to complicate my leaving (the office manager basically threatened me to the point of quitting), I would certainly be prepared to use any information I could to fight back.
Employment Standards Act:
http://www.gnb.ca/0062/acts/acts/e-07-2.htm
Your case would fall under number: 43(4)
43(4)An employer may, where no alternative employment is available, before or after commencement of the period referred to in subsection (1), require the employee to commence a leave of absence at the time when the duties of her position cannot reasonably be performed by a pregnant woman or the performance of the work of the employee is materially affected by the pregnancy.